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&hreegroupsofexperimehaveledthewayinlaboratorytestsofgravityiimes:oheUyofWashingtoheUyofColorado，andathirdatStanfordUheWashingtongrouptookapendulumwithteoitaoveradiskthatedaofholes.Thependulumwasfoundtotwistduetothemissinggravitatiohemissingmassihisallowedgravitytobemeasureddowntoscalesassmallasatwehofamillimetre.ThegroupsatColoradoandStanford，oherhahegravitationalfieldmass，ahisinformatiravityallthewaydownto140ofamillimetre.Theresultsoftheselaboratoryexperimentshavesofarallbeeirelytwithon’sinversesquarelawofgravity，evenontheverysmallestofscales.
erdistancescales，however，thereareavarietyofotherteststhatwemustsider.Toorderthedisofthesetestsseofallsiderdistaeres，uptoabouttoaboutafewkilometres.Thesescalesmayseemliketheyshouldbetheeasiesttoprobeastheyareclosesttothetypeofdistansiderinoureverydaylives.Theyare，however，quiteproblematic.
&hebesttsthathassofarbeenagravityovertheseeverydaydistancescalesisduetomeasuremeioaexperievariousheightsupatalltower.Towardstheendofthe1980s，ateamofstistsperformedthistestusire-highWTVDtarha.Thegravityexpeon’sinversesquarelawatdiferentpoioweriseasytocaldtheexperimeoodaccuracy.Atarouime，otherresearchersweremeasuriiohewaterinareservoirfilledtodiferentlevels.Ihisallowedthewatertobeweighedaon’sinversesquarelawofgravitytobetested.Furthertests，acoupleofyearslater，wereperformedbymeasuriionalattraofseawateratdifereheo.Theseexperimentsallproducedresultsthatweretwithon’sinversesquarelaw，andallatalevelofaccuracyofarou.
erscalesitstartstobeepossibletouseastronomicaldata，whichismuchmoreaccuratethantryihegravitatiohewaterinreservoirsaaweenamillionandafewhundredsofmillioresbeprobedbystudyisofmaesarouheorbitoftheMoosoftheplasarouheLAGEOSsatellites，laun1976and1992，areparticularlyusefulforexperimentsofthistype’asaretheorbitsoftheplas，whiclosedellipses（tofirstapproximatiohegravitationalattraisiveon’sinversesquarelaw.Observationsofallofthesebodies，as，haveshowhon’slawataccuraciesofbetweeinamillioinabillion.
&hereforehaveverygoodevideon’sinversesquarelaroximationtogravityoverawideraahesesgefromafraillimetre，tohundredsofmillioheaccuracyoftheexperimentsthatprobegravityoverthesedistancesvarieswildly，betartin1，000（forscalesofteoaboutoinabillion（fordistadtotheorbitsoftheplahisisagreatsuccess，butit’shestory.Let’son’slawandstartgthearisefromEiheory.
&hatprobeEiheory
&sdiscussedearlierhaveallisthatarefamiliartomostpeoplewhohavestudiedphysicsatschool:theass;theUyofFreeFall;aon’sinversesquarelawofgravity.Iion，Iwillisthatprobethemoreunfamiliargrouein’stheravity.Thespecificefectsthatresultfromthistheoryareoftensmallanddiculttodetectexperimeheless，theyareveryimportant，astheyprovideusthatbeusedtoviewaandgravityatadeeperlevel.
TherearealargenumberofefectsthatresultfromEiheoingtolimitmyselftfouroftheThesearetheanomalousorbitoftheplahebendinghtarouhetimedelaynalsastheypassbytheSun;andthebehaviyrosorbitarouhesearefourofthemostpromiicgravitatiobeobservedintheSolarSysteFurtherefects，whieappareremeastrophysients，willbedisChapter3.
&withtheanomalousorbitofMercury.Earlieroninthisbook，IstatedthatheoryexplainedKepler’sobservatiosfollowellipticalorbitsarouhisistruefleplahingsstarttogetabitmorepliwesidertheorbitsofmultipleplaahisisbecausethereexistgravitatioweesthemselvesandnotjustbetlaheSunindividually.Theseiarygravitationalforcesaresmallerbutmeasurable，aheefectofpullisoftheplasofwhatmightotherwisebeperfectellipses.
Physicistshaveknownabouttheefeterplaarygravitationalftime.Theseforbeeasilycalculatedwithiheory，andastronomershavemeasuredtheirtheorbitsoftheplasforturies.Infact，theexisteunewasdeduthemid-19thturybycarefullystudyiofUratotheSuofUraurbedalittlefromwhereastroedittobe，aioncouldbeexplaiherewasaslightlylargerplalebitfurtheroutintheSolarSysteUrbainLeVerrierandJohnsbothpredictedtheexistehisplain1845’andby1846ithadbeehiswasclearlyamomentousat.
Itthereforewasn’ttoomuchofasho，in1859，LeVerrieraheorbitofMercury（theplaheSuodeviatealittlefromthepaththatitectedtotake.Nodoubtbuoyedbyhispredioftheexisteune，LeVerrierpredictedthattheremustbeastillclosertotheSunthanMercury.Heeve’Vul.Thistime，however，thediscoveryofthepredietneveroccurred.Despitemuchefort，couldbeseeweenMerdtheSuheless，theanomalousorbitofMercurypersisted.ItseemedasifitsorbitwasbeingdraggedaroundtheSunbyagravitatiohadnoin.
&eryoftheanomalousorbitofMercurywassolvedin1915，notbythediscoveryofanynewmassivebodiesintheSolarSystem，butbyEinsteiioheory.Yousee，agtoEinsteiheory，theoniangravitationalforlyaroughapproximatioureofgravity.Aswellastheinversesquarelaw，Eihatthereshouldbenew，smallertributioionalforcelahysicalsystemdominatedbyalargemassatthetre，liketheSureoftheSolarSystem，Einsteincalculatedthatthelargestofthesenewforcesshouldvaryastheihedistaherefore，theeistotheSuhetributionfromthisnewforceshouldbe，relativetoon’sinversesquarelaw.
Mercurywas，andstillis，theowheSun，aeiationalfororeiialonMercury’sorbitthasofaherplas.
&eincalculatedthatMercury’sorbitshedaroundtheSura43ardspertury（1ardis1360ofadegree）.Thisisatinyamount，butitisenoughtobeobservedbyastroisalsotwiththeanomalousobservationsmadebyLeVerrier.SoEiheravityhaditsfirstobservationalsuccessasearlyas1915，byexplainingtheanomalousorbitoftheplaMercury.
ModersoftheorbitofMercuryaremuchbetterthaexistedihtury.theorbitsofalltheplahaccuracy，whichisessentialforgthedeviationofMeraradragcausedbyVenusalone，forexample，ismorethaoftheduetoEinstein’sgravity.Itspositionthereforeobeknhaccuracy.Thisissourtyinmoderionsthough.ThatdistiheuyassociatedwiththeshapeoftheSunitself.AionsfromaperfectlysphericalshapecauseefetheorbitofMercurythatcouldbefusedwithEinsteiatioheshapeoftheSunishardtokly，sothebestwetlydoistosaythattheanomalousorbitofMercuryistwithEiheorytoanaccuracyofabout1partin1，000.
&ionofMercury’sorbitisimpressivebutprobablydoesn’ttasapredi，becausetheanomalyitselfwasknownaboutloeinwasevenborn.WhatdoestasageioeihebendinghtaroundtheSunbyitsgravitatioreviouslyuailightshouldbeafectedbygravity，beewtravitationalforlybetweehavemass（whichlightdoesnot）.Thisambiguitywasremovedihehtsimplyfollowstheshortestpathsavailableinthecurvedspace-time，justlikeeverythihereforepredictedthatlightshouldbebeionalfieldsofmassiveobjects.
&ein’scalsshowedthatthedefleoflightwouldbegreatestforbeamsthatjustskimthesurfaassiveobjects.ThemostmassiveobjetheSolarSystemistheSun.ButthtthatpassesveryclosetotheSuforasolareclipse，otherwisetheSun’sownlightoverwhelmsthatofthemuterstars.ThefirstgoodopportuheideaofbendingstarlightcameaftertheendofWorldWarI，in1919.TheexpeditiohepositioarsheSuotestEiheravity，wasledbySirArthurEddington.
&oioheislandofPrínAfrica，wherethesolareclipsethatwasgoingtohappenthatyearwouldbetotal.Hemadecarefulmeasurements，usiographicplatesthatwereavailableatthetime.TheswerelessthaEddingtonsumeasuriioarsduringtheeclipse.Hefoundthattheyhadied，justasEinsteied，becausethetrajectoryofthelighthadbeehegravitatioheSuosweresuiEiheory，butonlywithinanaccuracyofarou.
On，moderionshaveimprovedthisresultdramatically.Thishasbeelybythefactthatthereare，tally，anumberhtobjeasquasars，whijusttherightpartoftheskytotestEinsteiiospassbehihede’eofthelightthattheyemitbemeasured.Severalmillioionsofthesequasarshavenowbeenmadeusingverylargebaseliers（thisisatypeoftelesixesthesignalsfromanumberofdifereocreateahigh-resolutioheresultofthisworkshowsperfesisteeialevelofaccuracyofabout1partin10，000.
AmoreretprediEiheoryisthetimedelaynalsthatpassbymassiveobjeereason，ittookuntil1964forstiststohisefeeeofEinstein’sgravity，butithasnowbeenmeasuredinanumberofdiferentsituations.ThesehaveiheobservatinalsthatbounetsastheyareabouttopassbehindtheSun，aswellasbylookingatthesignalsthatareemittedfromman-madeprobesastheydothesame.Thefirstofthesemethodshasthebehepositiosbeingverywellknowrajecteasytopredict.Thisstabilitymakesthemaget，buttheimperfetheirshapeeproblemsihere’eals.Man-madeprobes，oherhaveryeasilypredictablesignals，buttheirtrajectoriesewhatless.
RadietsthathavebeeomeasurethetimedelayefecludetheplasMerdVenus，andthespaariners6and7，Voyager2，theVikingMarsLanderaheiprobe.Themostredmostaccurate，oftheseobservationswasmadeusingi.TheprimarymissionofthisspacecraftwastoobserveSaturn，butitsrelevahestudyofgravitysbestservedin2003，whenitwasahadedtheexisteimedelayefeaccuracyof1partin100，000.Thisectaatioein’stheory，andonewhichwasachievedathigheraccuraypreviousexperiment.Thisart，duetotheobservationsnalsbeiiplefrequencies，whichallowedinterfereheSun’satobeextracted.
’smoveontothefiIwanttodisthisse:thebehaviyrosorbitarouh.Agyroscopeisessentiallyaspinningtop，whosetralaxisisallowedtopointiitoEiheravity，thereshouldbetwobeobservedeisputinorbitarouhefirstoftheseisadireofthegyroscope’saxisofrotation，asitorbitstheEarth.Thisefeasgeodeticpre，isduetothecurvatureofspace-timearouhesedefeing，aherotatiohefectivelypullingspadwithitasitrotates.Thisisaypeofgravitatioion，andsoitisofgreatioseeifitedexperimentally.
AlthoughtheprediiwasmadeonlyafewyearsafterEinsteinpublishedhisheravity，ittookuntilthe1960sbeforetheefegyrosorbitwascalculated，aalobservatiohemselvesdidnottakeplatilthe21sttury.TheLAGEOSsatelliteworkprovidedaionibymeasuringthetheorbitofitssatellitesastheywentarEarth.Thelong-awaitedgyroscopeexperimenterformedbyamissioyProbeB，in2011.Thegeodeticpredframe-draggisweremeasuredbythisexperimentwithaccuraciesofabout0.3perd20pert，respectively.TheaccuracyofthedisfromtheLAGEOSsatellitesareestimatedatbetert.Allresultswereonfoueein’stheory.
SotheoverallpictureweareleftwithisveryengforEiheravity.Thefoundationalassumptionsofthistheory，suchastheassandtheUyofFreeFall，havebeeremelyhighaccuraversesquarelawthatformedthebasisofheory，andwhichisagoodfirstapproximatioein’stheory，hasbeehesub-millimetrescaleallthetoastrophysicalscales.WearealsonowinpossessionofanumberofaccurateexperimehatprobethetihatresultfromEiheoryspecifically.Thisdataallowsusdirectexperimentalinsightiioteraureofspace-time，andallofitissoodagreemeeiions.Thisisatrulyspectaatiowasborofalmht.Eiedtomakeatheravitythatatibleeedoflightthatwasmeasuredtobethesamebyeveryobserver.Hedidso，andwehavehenumerouscesofhisrevolutioureoftheUniverse.
Thisishesth:Einstein’sgravityhasmaasequences，illdisthefollo
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